Second Order Nonlinearity
The second order nonlinear polarisation of lithium niobate can be written as,
The 2-D matrix describes the non-susceptibility tensor χ(2). For 5% MgO doped lithium niobate (MgO:LN) at 1064nm, d31=4.4pm/V, d33=25pm/V[1].
The highest nonlinear coefficient is d33=25pm/V, which corresponds to interactions that are parallel to the z-axis, i.e. type-0 phase matching. In other words, all interactive waves must be e-polarized in order to achieve the highest conversion efficiency. All of our crystals are designed to access this d33 coefficient. For periodically poled MgO:LN, the effective nonlinear coefficient deff is typically 14pm/V.
NOTE: Covesion can offer custom crystals for type I or type II interactions, for example for entangled photon systems for the generation of orthogonally polarized pairs.
Refractive Index
The temperature dependent refractive index is described by the Sellmeier equation:
Where the temperature dependent parameter f is defined as,
Where T is temperature in °C
and the Sellmeier coefficients are, an optical resonator, also known as an optical parametric oscillator (OPO), the efficiency can be significantly enhanced.
| Sellmeier Coefficient | 5% MgO:LN [2] | Undoped LN [3] |
|---|---|---|
| a1 | 5.756 | 5.35583 |
| a2 | 0.0983 | 0.100473 |
| a3 | 0.2020 | 0.20692 |
| a4 | 189.32 | 100 |
| a5 | 12.52 | 11.34927 |
| a6 | 1.32E-02 | 1.5334E-02 |
| b1 | 2.860E-06 | 4.629E-07 |
| b2 | 4.700E-08 | 3.862E-08 |
| b3 | 6.113E-08 | -8.9E-09 |
| b4 | 1.516E-04 | 2.657E-05 |
Using these parameters in the Sellmeier equation, you can calculate the refractive index variation with wavelength and temperature. The table below has a few examples.
| Temperature | 532nm | 780nm | 1064nm | 1550nm | 3500nm |
|---|---|---|---|---|---|
| 30°C | 2.2260 | 2.1715 | 2.1496 | 2.1320 | 2.0732 |
| 100°C | 2.2485 | 2.1929 | 2.1708 | 2.1530 | 2.0938 |
| 150°C | 2.2673 | 2.2108 | 2.1884 | 2.1705 | 2.1110 |
PPLN has a high index of refraction that results in a ~14% Fresnel loss per uncoated surface. To increase transmission through our crystals, the crystal input and output facets are AR coated, thus reducing the reflections at each surface to less than 1%.
Transmission
MgO:LN and LN have very similar transmission curves and are highly transparent from 400-4000nm. Material absorption occurs below 400nm and above 4000nm where PPLN can still be used as long as the losses can be overcome. For example, a pulsed mid-infrared OPO generating 7.3um has been demonstrated in PPLN[4], although more commonly, PPLN-based OPOs are often operated up to 4.5-5um. Similarly, for the UV region, generation at 386nm[5] and 370nm[6] has been demonstrated using 3rd order QPM in MgO:PPLN.
Work by Schwesyg et al. have analysed the absorption losses of MgO:LN between 300 and 2950nm[7]. Their data (shown below) provides an accurate measurement of the absorption coefficient between 400-800nm. Their experiment also found no measurable absorption bands between 800-2000nm.
The figure below shows the transmission curves of LN and MgO:LN measured by Covesion, showing the roll-off of transmission for both materials. The measurement includes Fresnel reflections off both input and output facets of the measured samples, which accounts for a loss of approximately 30% due to Fresnel reflections.
NOTE: There is an OH-absorption band at 2826nm with a measured absorption coefficient of 0.088cm-1 [7].
MgO:PPLN vs undoped PPLN
Undoped PPLN is usually operated at temperatures between 100°C and 200°C, to minimize the photorefractive effect that can damage the crystal and cause the output beam to become distorted. Since the photorefractive effect is more severe in PPLN when higher energy photons in the visible part of the spectrum are present, it is especially important to use the crystal only in the recommended temperature range.
The addition of 5% MgO to lithium niobate significantly increases the optical and photorefractive resistance of the crystal while preserving its high nonlinear coefficient. With a higher damage threshold, MgO:PPLN is more suitable for high power applications. It can also be operated from room temperature up to 200°C, significantly increasing the wavelength tunability of the device. Moreover, in some special cases, the MgO:PPLN can be operated at room temperature and without the need for temperature control e.g. Our MSHG1550-0.5-1 (1mm long) can be used for generating 780nm from 1560nm femtosecond fiber laser.
Power Handling and Damage Threshold
Lifetime testing of our crystals is an on-going process at Covesion. Using a 10W 1064nm CW laser, we have generated 2.2W at 532nm. With a pump intensity of >500KW/cm2 and operating temperature of 35degC, our PPLN maintained the 2.2W SHG output power over a period of 2000hrs, with no signs of damage to the crystal and no evidence of beam distortion due to photorefraction.
The damage threshold of MgO:PPLN or PPLN depends on wavelength as well as whether the source is CW or pulsed. In the CW regime, the threshold depends on the intensity and is lower when visible wavelengths are involved. For pulsed sources, the damage threshold depends on wavelength, pulse duration, average power and the repetition rate. Often the damage threshold will be higher for low repetition rate sources.
If you think that you are working close to the damage threshold, then a good tip is to test the damage threshold in an unpoled region of the crystal. Covesion crystals have a standard width of 10mm, but the poled gratings cover a maximum width ~7mm. You can use the unpoled areas to carefully test for damage as long as it is still within the AR coated region.
Note: The damage threshold in a poled region will be lower if you are generating visible wavelengths. Always increase the pump power gradually, whilst monitoring the beam for any distortions or a sudden drop in power.
The table below shows a collection of data from Covesion and from customers showing the power handling or damage thresholds under various regimes. We are continuously working together with our customers to increase the amount of information available on crystal damage thresholds.
Regime | Peak Intensity/ Energy Density | Damage? | Notes |
|---|---|---|---|
| CW | 500KW/cm2 | N | 1064nm, 10W, SHG (Covesion) |
| CW | 500KW/cm2 | N | 1560nm, 30W, (Australian National University[8]) |
| CW | 200kW/cm2 | N | 532nm, 2.2W, (from 1064nm SHG) (Covesion) |
| ns | 100MW/cm2 or 2J/cm2 | Y | 1064nm, ~30um period, single pass, 10-20ns, 21Hz, (Covesion) |
| ps | 100MW/cm2 | N | 1060nm OPO, 20ps, 115MHz, 24W (ORC Southampton,[9]) |
| ps | 1.5GW/cm2 | N | 1064nm OPG for MIR: 7ps, 400Hz |
| ps | 1.8MW/cm2 | Y | 530nm OPO, 20ps, 230MHz, 500mW, (ORC Southampton,[10]) |
| ps | 7.5MW/cm2 | Y | 530nm OPO, 20ps, 230MHz, 1W->100mW chopped, (ORC Southampton,[10]) |
| ps | 468MW/cm2 | N | 1064nm, 7ps, 17W, 80MHz, (National University of Singapore[11]) |
| fs | 4GW/cm2 | Y | 1550nm, 200fs, 200mW, 80MHz, SHG |
Damage Mechanisms
The Photorefractive Effect
Under conditions of high intensity, LiNbO3 and MgO: LiNbO3 are prone to the photorefractive effect, which is an optically induced change in refractive index. (N.B. The threshold is higher for MgO: LiNbO3).
In a region of high optical intensity, electrons are released as free carriers and then redistribute in an area of lower optical intensity. This causes a spatially varying refractive index within the material that can be observed as beam distortions. This can result in permanent damage to the crystal. However, under some circumstances, if the effects are small the damage can be reversed by heating the crystal to 200°C for a couple of hours to allow all the charge carriers to re-diffuse.
If you are working near the damage threshold, it is recommended that you operate at high temperatures between 150-200°C.
Green Induced Infrared Absorption
Green Induced Infrared Absorption, or GRIIRA, is an effect where the presence of green light allows infrared to be absorbed. This causes local heating which can offset the phase matching temperature of your interaction, but it can also eventually lead to crystal damage.
The mechanism for GRIIRA comes from the creation of polarons from crystal defects such as, Nb ions occupying Li ion sites (known as antisite defects), and Fe ion impurities. Doping lithium niobate with MgO reduces the onset of GRIIRA, as it allows Mg ions to replace the Nb antisite defects.
IR absorption due to blue light also occurs by the same mechanisms, and is known as BLIIRA (blue induced infrared absorption).
References
1. Compact blue-green lasers”, W. P. Risk, T. R. Gosnell and A. V. Nurmikko, Cambridge University Press, 2003
2. Gayer et al., Applied Physics B 91, 343-348 (2008)
3. D.H.Jundt, Optics Letters V.22 N.20 p.1553-1555 (1997)
4. M. A. Watson et al., Optics Letters, vol. 27, no. 23, pp. 2106–8, ( 2002)
5. R.T. White et al., Applied Physics B: Lasers and Optics, 77(6-7), 547–550 (2003)
6. J. Kim, et al., 2013 IEEE Photonics Society Summer Topical Meeting Series (pp. 183–184) (2013)
7. J. R. Schwesyg et al., Advances in Optical Materials, AIThE3, (2011)
8. S. S. Sané et al., Optics Express, vol. 20, no. 8, pp. 8915–9, (2012)
9. F. Kienle et al., Optics Express, vol. 18, no. 8, pp. 7602–10, (2010)
10. F. Kienle et al., Journal of the Optical Society of America B, vol. 29, no. 1, p. 144, (2011)
11. P. K. Upputuri and H. Wang, Applied Physics B, vol. 112, no. 4, pp. 521–527, (2013)
